1. Field of the Invention
The present invention relates generally to stabilizing a property of the output of a laser system, and more particularly to an optical system including a laser where finer tuning of the output power is required than is provided by standard, built-in digital set points.
2. Description of the Related Art
In recent years, medical and industrial applications using pulsed laser systems have proliferated. As the lasers have become more reliable and commonplace, there has also been a greater emphasis on improving control of the laser parameters so as to improve outcomes in practical settings. Providing the required controls presents a greater challenge as increasingly complex laser systems are being introduced into applications which place stringent demands on performance and operating lifetime even as preferred devices are required to be more compact and cost effective. Ultra-fast lasers, build-up cavities involving resonant frequency doubling, systems including optical parametric conversion devices and multiple harmonic modules and high power fiber lasers are all examples of complex laser systems requiring sophisticated controls to perform their intended functions.
Ultrashort pulse lasers have, in particular, been promoted as an effective new tool for a variety of medical and industrial applications, and especially where small interactions zones, fine feature sizes and limited collateral damage are considered highly beneficial. Examples include metrology measurements, two-photon microscopy, material processing, stereolithography and corneal sculpting procedures. In the case of material processing applications ultrafast lasers exploit localized laser induced breakdown mechanism to provide submicron processing capability. Some applications exploit the ability of ultrafast lasers to ablate surface regions that are even smaller than their minimum, diffraction limited spot size. Many micro-machining, inscription and hole drilling procedures have been proposed that take advantage of the high degree of precision provided by ultrafast interactions. Examples include: drilling holes with sub-wavelength pitch as may be used to produce photonic crystals as described in U.S. Pat. No. 6,433,305, removal of biological and other types of material incurrring minimal collateral damage and greatly increased cut quality as taught in U.S. Pat. No. 5,720,894, precise surface ablation in either opaque or transparent materials as described in Pat. No. 5,656,186 and No. 6,333,485 and inscription of micro patterns in various materials.
The efficacy of micro-machining procedures carried out with ultrashort pulse lasers depends in a large measure on the precision of controls provided by the system of the key output laser parameters including power, pulse energy and/or pulse width. In particular, controlling and stabilizing the output power are essential to the precision with which micro-holes can be drilled, micro-patterns can be inscribed or clean repeatable cuts are performed. Procedure repeatability and high throughputs are especially important considerations for virtually all industrial, biological and surgical applications which contemplate the use of ultrafast lasers.
Another especially good example of an application requiring a high degree of control is provided by emerging metrology applications such as the ultrasonic short pulse technique (known by trade name MetaPULSE) successfully developed into a semiconductor inspection tool at Rudolph technologies. The technique uses femptosecond pulses to produce ultrasonic echoes which are analysed to derive the thickness of single or multi-layer metal films used in integrated circuit manufacturing. at high throughput rates. With metal layers ranging from under 20 Å to over 5 μm, high precisions with better than 1-2% repeatability are required along with high throughput rates. Precise control of key laser parameters is therefore essential characteristic for this application. In particular, variations in power can contribute to nonuniformities in thickness measurements which can compromise the measurements.
In many of foregoing applications, it is required that the laser be capable of hands-off reliable operation for prolonged periods of time in an industrial or medical setting. At the same time during the time the output laser beam is coupled to a work piece, the laser must provide power levels and other operational characteristics that are as constant as possible and be free of long term drift or unpredictable power instabilities.
Generally, it is known that uncontrolled fluctuations in power or other laser parameters such as the pulse width, wavelength or beam divergence lower the accuracy of the laser interactions with a target material and compromise the system performance. Whereas methods of stabilizing operating laser parameters are known in the art, many such techniques require numerous additional components and are too complex to implement in an industrial setting especially where reliable throughputs and space considerations are paramount. It is highly desirable to provide a laser system with improved reliability and stabilized output control features on a fine scale using the most expeditious and cost effect means.
Typically, the more complex laser systems that are the subject of the present invention comprise at least two or more key subsystems, each of which may be a laser cavity or optical system. In this case changing parameters of an output beam which is the one delivered to the target requires controlling an existing input system or subsystem with its own fully designed control electronics and drivers.
For example, one subsystem may be a pump laser such as a commercially available diode pumped green laser that can be used to pump another subsystem, such as a Ti:sapphire laser. Other possible subsystems might include an optical parametric converter or a Raman shifter to provide a fixed set of wavelengths. In still other examples the optical subsystem may include build up cavities for resonant harmonic conversion or an injection seeded amplifier.
In all of these cases, controlling and adjusting the output power of a laser consisting of one or more complex subsystems can be a major issue. For example, requiring fine control of selected properties of an output beam such as pulsewidth or the power can present an issue when the driver electronics are controlled digitally. Any digital controller has a finite number of quantized command levels. These quantized command levels are often too coarse to provide the high-resolution control of the output power of the system as required to meet the needs of certain applications. This can be an even more of an issue when there are additional noise and/or bandwidth specifications on the output of the system.
There is a need for techniques that can provide a high degree of control of selected properties of the output from optical systems which may include one or more laser subsystems. There is a particular need for cost effective techniques that allow making smaller adjustments to the output power of the system than is currently possible with digital set points or some other quantized means. There is further need to be able to make these adjustments sufficiently fast to maintain adequate throughput rates for the applications contemplated by the system.